Carbon nanotubes are fullerene-related structures which consist of graphene cylinders closed at either end with caps containing pentagonal rings. They were discovered in 1991 by the Japanese electron microscopist Sumio Iijima who was studying the material deposited on the cathode during the arc-evaporation synthesis of fullerenes. He found that the central core of the cathodic deposit contained a variety of closed graphitic structures including nanoparticles and nanotubes, of a type which had never previously been observed. A short time later, Thomas Ebbesen and Pulickel Ajayan, from Iijima""s lab, showed how nanotubes could be produced in bulk quantities by varying the arc-evaporation conditions. This paved the way to an explosion of research into the physical and chemical properties of carbon nanotubes in laboratories all over the world.
A major event in the development of carbon nanotubes was the synthesis in 1993 of single-layer nanotubes. The standard arc-evaporation method produces only multilayered tubes. It was found that addition of metals such as cobalt to the graphite electrodes resulted in extremely fine tube with single-layer walls. The availability of these structures should enable experimentalists to test some of the theoretical predictions which have been made about nanotube properties.
An alternative method of preparing single-walled nanotubes was described by Smalley""s group in 1996. Like the original method of preparing C60, this method involved the laser-vaporization of graphite, and resulted in a high yield of single-walled tubes with unusually uniform diameters. These highly uniform tubes had a greater tendency to form aligned bundles than those prepared using arc-evaporation, and led Smalley to christen the bundles nanotube xe2x80x9cropesxe2x80x9d. Initial experiments indicated that the rope samples contained a very high proportion of nanotubes with a specific armchair structure. Subsequent work has suggested that the rope samples may be less homogeneous than originally thought. Nevertheless, the synthesis of nanotube ropes gave an important boost to nanotube research, and some of the most impressive work has been carried out on these samples.
Nanotubes, depending on their structure, can be metals or semiconductors. They are also extremely strong materials and have good thermal conductivity. The above characteristics have generated strong interest in their possible use in nano-electronic and nano-mechanical devices. For example, they can be used as nano-wires or as active components in electronic devices.
Beginning with the early work of Binnig, Rohrer and co-workers, the utility of a direct, physical connection between the macroscopic world and individual objects on the nanoscopic scale has become increasingly evident, both for probing the nature of these objects and as a means of direct manipulation and fabrication. While good progress has been made in controlling the position of the macroscopic probe to sub-angstrom accuracy and in designing sensitive detection schemes, there remains the problem of the tip. Ideally, the tip should be of at least the same molecular precision as the nanoscale object to be probed, and it should maintain this perfection reliably in day-to-day practical use not only under high vacuum but also in air and when probing under water. The currently available tips for scanning force microscopy, SFM, or scanning tunneling microscopy, STM, do at times achieve sub-nanometer resolution, they seldom survive a direct xe2x80x9ctip crashxe2x80x9d with the surface. Further, it is rarely clear just what the atomic configuration of the tip actually is when the image is taken. Carbon nanotubes, particularly those which are effectively fullerenes of macroscopic length in one dimension but still intrinsically nanoscopic with molecular perfection in the other two dimensions, may offer the solution to this tip problem.
The atomic force microscope (AFM), or scanning force microscope (SFM) was invented in 1986 by Binnig, Quate and Gerber. Like all other scanning probe microscopes, the AFM utilises a sharp probe moving over the surface of a sample in a raster scan. In the case of the AFM, the probe is a tip on the end of a cantilever which bends in response to the force between the tip and the sample. The first AFM used a scanning tunneling microscope at the end of the cantilever to detect the bending of the lever, but now most AFMs employ an optical lever technique.
The cantilever obeys Hooke""s Law for small displacements, the interaction force between the tip and the sample can be found. The movement of the tip or sample is performed by an extremely precise positioning device made from piezo-electric ceramics, most often in the form of a tube scanner. The scanner is capable of sub-angstrom resolution in x-, y- and z-directions. The z-axis is conventionally perpendicular to the sample.
Compression occurs when the tip is over the feature trying to be imaged. It is difficult to determine in many cases how important this affect is, but studies on some soft biological polymers (such as DNA) have shown the apparent DNA width to be a function of imaging force.
Interaction forces between the tip and sample are the reason for image contrast with the AFM. However, some changes which may be perceived as being topographical, may be due to a change in force interaction. Forces due to the chemical nature of the tip are probably most important here, and selection of a particular tip for its material can be important. Chemical mapping using specially treated or modified tips is another important aspect of current research.
The aspect ratio (or cone angle) of a particular tip is crucial when imaging steep sloped features. Electron beam deposited tips have been used to image steep-walled features far more faithfully than can be achieved with the common pyramidal tips.
The remarkable structural and mechanical properties of single-walled carbon nanotubes (SWNT) make them ideal atomic force microscopy (AFM) probes. Methods for their fabrication based on chemical vapor deposition (CVD) favor the production of SWNT bundle probes, resulting in poorer resolution than what could be achieved with individual SWNT.
Currently, AFM""s limiting technology is arguably its most importantxe2x80x94the structure of the probe tip that interacts with the sample. AFM tips are prepared by microfabrication and are pyramidal with ca. 5 nm (silicon) or ca. 50 nm (silicon nitride) radii of curvature at the tip apex, and cone angles of ca. 30 degrees. In many imaging applications, sharper and higher aspect ratio tips are needed. In addition, the atomic structure of the tip is not well characterized and can even change while imaging. Several techniques have been developed to improve tips including FIB etching, electron beam deposition, diamond-like coatings, and the use of small asperities naturally occurring at the tip. While these represent improvements for certain samples and imaging conditions, none provide a tip structure with both high aspect ratio and sub-nm radius of curvature while being chemically stable and mechanically robust.
Previously, nanotube probes have been prepared by manual assembly of pre-formed nanotube material and commercial AFM tips by micromanipulation while viewed with an optical microscope. Although this procedure is laborious and slow, it allowed the early development of nanotube tips including the observation of nanotube buckling, high resolution and high aspect ratio imaging, and chemical modification of the tips for chemical force microscopy. The limitations of manually assembled nanotube tips were addressed by direct growth of nanotubes at the tip apex by chemical vapor deposition (CVD). This process provides individual multiwalled nanotube tips and single-walled nanotube (SWNT) bundle tips, and makes their fabrication much simpler since it can be carried out on many tips at once. The ultimate high-resolution tip would be an individual SWNT tip rather than a bundle.
In one aspect, the invention provides for a method of fabricating SWNT probes for use in atomic force microscopy, comprising the steps of: growing SWNTs on a substrate using chemical vapor deposition; imaging said substrate using an atomic force microscope comprising a tip; and attaching one of said SWNTs to said tip, thereby producing a tip bearing a SWNT. In one embodiment, the SWNTs are deposited normal to the surface of said substrate. In a further embodiment, the substrate is a silicon wafer.
In another embodiment, growing the SWNTs on a silicon wafer comprises the steps of: depositing on said wafer a metallic catalytic material; placing said silicon wafer in a CVD furnace; and exposing said silicon wafers to a gaseous atmosphere comprising a carbon containing gas. In a further embodiment, the metallic catalytic material is selected from the group consisting of metals, metal oxides, metallic salts, and metallic particles.
In an embodiment, the metallic catalytic material is in solution. In a further embodiment, the metallic catalytic material is selected from the group consisting of ferric salts, nickel salts, cobalt salts, platinum salts, molybdenum salts, and ruthenium salts. In a particular embodiment, the metallic catalytic material is ferric nitrate. In a further embodiment, the solution comprises an alcohol. In an even further embodiment, the alcohol is selected from the group consisting of methanol, ethanol, and isopropanol. In yet a further embodiment, the alcohol is isopropanol.
In one embodiment, the carbon containing gas is ethylene. In a further embodiment, the carbon containing gas is ethylene, the metallic catalytic material is ferric nitrate, and the alcohol is isopropanol.
In one embodiment, imaging the substrate further comprises applying a pulsed electric field.
In another embodiment, growing the SWNTs on a silicon wafer comprises the steps of: treating said silicon wafer with metallic colloid particles; placing said silicon wafer in a CVD furnace; and exposing said silicon wafers to a gaseous atmosphere comprising a carbon containing gas. In an embodiment, the metallic colloid is selected from the group consisting of iron colloids, nickel colloids, cobalt colloids, platinum colloids, molybdenum colloids, and ruthenium colloids. In a particular embodiment, the metallic colloid is an iron colloid.
In an embodiment, the carbon containing gas is ethylene. In another embodiment, the metallic colloids have diameters of about 3-15 nm. In yet another embodiment, the SWNT has a diameter from about 2 nm to about 13 nm. In yet another embodiment, the SWNT has a diameter from about 2 nm to about 9 nm. In another embodiment, the SWNT has a diameter from about 3 nm to about 5 nm.
In an embodiment, the method further comprises a tip bearing an adhesive. In another embodiment, the method further comprises the step of heating said tip bearing a SWNT. In another embodiment, The method comprises treating the tip bearing a SWNT with an electromagnetic field.
In another embodiment, a method of growing carbon nanotubes comprising the steps of: providing a substrate; treating said substrate with a metallic colloid solution; placing said substrate in a CVD furnace; and exposing said substrate to a gaseous atmosphere comprising a carbon containing gas, thereby growing a carbon nanotube on said substrate. In a further embodiment, the carbon nanotube is a SWNT. In a further embodiment, the carbon nanotube is a MWNT. In a further embodiment, the substrate is a silicon wafer. In a further embodiment, the metallic colloid is selected from the group consisting of iron colloids, nickel colloids, cobalt colloids, platinum colloids, molybdenum colloids and ruthenium colloids. In a particular embodiment, the metallic colloid is an iron colloid. In a particular embodiment, the carbon containing gas is ethylene. In another particular embodiment, the metallic colloids have diameters of about 3-15 nm. In another particular embodiment, the solution comprises an organic solvent. In yet another embodiment, the solution comprises toluene.